WIP: feat(controller): Use etcd directly for scheduling

Discover cluster hosts using a etcd discovery url instead of hardcoding a list of hosts

Partially fixes #1727

@carmstrong we discussed about this a while ago and actually it seems to be relatively easy. Maybe it could be enhanced further by using the last updated host instead of randomly trying one.
The relevant test just refutes that the command does not return an empty string. It should actually be looking for `hello` instead: https://github.com/deis/deis/blob/master/tests/apps_test.go#L101

ping @mboersma